This invention relates to power generation assemblies, and apparatus for use therewith. More specifically, this invention relates to (a) a floating power generation assembly; (b) a process for placing a floating unit on water, this process being especially, although not exclusively, intended for use in deploying certain components of the floating power generation assembly of the invention; and (c) a multiple wind turbine assembly.
As concern over the environmental consequences of conventional power plants, including their carbon dioxide emissions, has increased in recent years, greater attention has been focused upon so-called “green” or environmentally advantageous power plants which use renewable sources of energy and do not cause substantial emissions of carbon dioxide or other pollutants. Potential green power plants include photovoltaic plants, which generate energy from sunlight, and plants which derive energy from tides, ocean currents and wave action.
One type of green power plant which has already been shown to be commercially viable is the wind turbine or windmill. So-called “wind farms” having multiple wind turbines have been constructed in several parts of the world and have made significant contributions to electricity production. In 2002, total wind farm capacity in the European Union was about 23,000 MW, and in the United States about 5,000 MW.
Unfortunately, although wind farms are environmentally advantageous in the sense of not emitting pollutants, they can create opposition due to their visual impact. To be economically viable, wind farms need to be situated where high average wind velocities are expected. On land, such sites are often on mountain ridges or on flat plains, and in either location conventional wind farms, using individual rotors 30 meters or more in diameter mounted on masts about 30 meters high, are conspicuous for miles. Furthermore, such mountain ridges or plains are often in areas celebrated for their natural beauty and such wind farm projects can face stiff opposition, resulting in higher project costs.
Accordingly, interest has recently shifted to off-shore wind farms. The first such off-shore wind farms have been established in shallow water (typically 15 meters or less deep) close to shore, and the equipment used has been essentially the same as in land-based wind farms, with the masts supporting the rotors mounted on the seabed and lengthened as necessary to keep the rotors at the desired height above the water. However, such shallow water wind farms have attracted the same types of controversy as land-based wind farms. For example, a recent proposal to place a large wind farm of more than 100 units in Nantucket Sound off the coast of Massachusetts has led to objections that the wind farm will have too much visual impact on ocean views. It has also been alleged that the rotors may kill or injure substantial numbers of birds.
Public controversy relating to wind farms would be reduced by moving off-shore wind farms a greater distance off-shore, although the maximum distance off-shore where wind farms can be located is limited by the expense of the undersea high voltage cables required to bring the electricity generated on-shore; such cables can incur very significant costs. Moreover, the choice of suitable off-shore locations for wind farms, even relatively close to shore, is limited by water depth. If wind farms are required to operate in deeper waters, say 100-200 meters, as the water depth increases, it becomes increasingly impracticable, from both engineering and economic view points, to continue with seabed mounted masts bearing single large rotors. Clearly at some point, it becomes necessary to base the wind farm upon one or more floating or tension leg platforms. However, to justify the high costs of deeper water wind farms, such farms will typically be required to have high power outputs, and the conventional type of single mast/single rotor wind turbine with very large rotors may not be well adapted for mounting upon a floating or tension leg platform. In one aspect, this invention seeks to provide a novel type of wind turbine assembly. The wind turbine assembly of the present invention may be useful in off-shore wind farms or other contexts, for example some land-based wind farms.
The present invention also relates to improvements in off-shore power generation assemblies, especially wind farms, to enable such assemblies to be sited in deep water without mounting a rigid structure on the sea bed or other underwater solid surface. Finally, this invention relates to a process for placing floating units on water, this process being especially intended for use in the deployment of the off-shore power generation assemblies of the present invention.
The power generation assemblies or wind farms described in the aforementioned application Ser. No. 11/938,138 use vertically-free-floating (“VFF”) units, that is to say buoyant units which float freely without any tension leg connecting them to the sea bottom; the cables used in the wind farms are used to provide horizontal tension support from multiple directions, thus making the VFF unit more stable against horizontal forces. The cables also ensure that the VFF units do not drift away from the predetermined locations, and maintain correct position relative to each other. While such VFF units can produce good results, the need to ensure that the center of gravity of each floating unit is a substantial distance below the water surface and that each unit has a substantial metacentric height (the distance between its centers of gravity and metacenter) of several meters, coupled with the need to mount a relatively heavy wind turbine and generator high above the water surface, means that in practice the VFF units must be heavy, typically of the order of several hundred to a couple of thousand tonnes. Such heavy VFF units require large quantities of construction materials and hence are costly to produce, especially in view of the recent substantial increases in the costs of construction materials such as concrete and steel due to increased energy costs.
It is known that wind turbines and other devices which it is desired to use at sea can be mounted on a tension leg platform (TLP). A tension leg platform comprises a buoyant body connected to at least one, and typically three or more, cables or similar connecting devices which are anchored to the sea bed. The cables are kept under substantial tension, and the buoyant body is effectively tethered to the seabed.
TLP's can be made more stable than VFF units of the same height, but, as discussed in more detail below with reference to FIGS. 46 to 48, they can undergo catastrophic tipping under large horizontal forces than VFF units, and this susceptibility to horizontal forces poses problems with mounting apparatus which requires locating units of substantial weight, such as rotors and generators, on TLP's at substantial distances above the water surface, since the mounting of substantial weight high above the water surface exacerbates the tendency of TLP units to suffer catastrophic tipping.
It has now been found that if some or all of the VFF units in the wind farms described in the aforementioned applications are replaced by TLP's substantial advantages accrue; the TLP's can be lighter and less expensive than similar VFF units, while the interconnections between the floating units provided by the anchors and cables of the wind farm itself (as opposed to anchors and cables associated with any individual TLP) reduce the sensitivity of the TLP's to tipping and horizontal forces and hence render the TLP's a more stable mounting for rotors and generators used for power generation.